In an internal combustion engine having an electronically controlled throttle, typically, a throttle opening is set based on, for example, an accelerator operation amount achieved by a driver and a throttle is operated according to the set throttle opening. If, at this time, a delay time is set before the throttle is operated after the throttle opening is set, an actual throttle opening changes at timing later than the set throttle opening for the set delay time. Such a throttle delay control allows a future throttle opening to be estimated for the delay time involved using the throttle opening before the delay control. The throttle delay control is effective in enhancing controllability of an air-fuel ratio to thereby improve exhaust emission performance for the following reason. A cylinder intake air quantity is defined at timing at which an intake valve is closed. The throttle delay control estimates with high accuracy the throttle opening at that particular timing to thereby allow a fuel injection quantity to be calculated based on the cylinder intake air quantity obtained from the estimated throttle opening.
A time delay is, however, introduced in torque response of the internal combustion engine to torque requirements by a driver. FIG. 6 is a diagram showing a relationship between a response time (e.g. 63% response time) and an engine speed when the torque response of the internal combustion engine to torque requirements is represented by a first order lag characteristic. A solid line in FIG. 6 shows the relationship between the response time and the engine speed when the throttle opening is set based on the accelerator operation amount and the throttle is operated without the delay control, and a dotted line shows the relationship between the response time and the engine speed achieved when the delay control is performed. From a standpoint of drivability, desirably, the response time is as short as possible. In addition, torque control is used in various vehicle control systems including a vehicle stability control system (VSC), a traction control system (TRC), and an electronic controlled transmission (ECT). To make effective the torque control by these control systems, desirably, the response time is as short as possible.
As observed above, a need exists in the throttle delay control to balance between air-fuel ratio controllability and torque response performance. JP-A-2004-150275, for example, focuses on this need. This publication discloses a technique in which, when an urgent valve closing request is made in VSC control or TRC control, the throttle delay control is interrupted and, instead, the throttle is immediately driven toward a target opening.
In the technique disclosed in the abovementioned Patent Document, however, there is still room for improvement in balancing between air-fuel ratio controllability and torque response performance. Referring to FIG. 6, should an urgent valve closing request be made, the technique disclosed in the above Patent Document allows the response time to be shortened by interrupting the delay control. However, the higher the engine speed, the shorter the response time. Understandably, therefore, a sufficiently high torque response can be obtained without needing to interrupt the delay control, if the engine speed is high when the urgent valve closing request is made. If, on the other hand, the engine speed is low when the urgent valve closing request is made, a sufficient torque response may not be obtained even by interrupting the delay control.